In producing photographic film, it is necessary to manufacture photographic emulsions capable of providing a developable image. Such photographic emulsions include gelatin solutions containing silver halide or other auxiliary materials used in manufacturing photographic products (e.g. color couplers). In producing silver halide emulsions, the process steps of chemical and spectral sensitization, ripening and post-ripening are well known. Once the emulsion has been post-ripened and sensitized to the desired level, the emulsion is chilled and stored in a gelled state. This highly sensitized form of emulsion is metastable and must be prevented from further ripening to a more stable state which is fogged and photographically useless.
When a gelled emulsion is to be utilized for producing photographic film, the gel is melted and then coated on a substrate. Once coating is completed, the emulsion is again chilled to a gel and then dried.
Traditionally, liquid, photographic emulsions are poured into containers which are placed in a refrigerated room so that the emulsion hardens into a gel. This cooling technique causes the emulsion closest to the surfaces of the container to gel first, while interior portions of the emulsion gel later. Unfortunately, the gelled emulsion adjacent to the container surfaces insulates interior portions of the emulsion and, consequently, further delays gelling at such locations. This delay adversely affects the uniformity of emulsions, because, when a long gelling period is required, the emulsion settles and becomes non-homogeneous in various parts of the container when finally gelled. Another problem with this gelling technique is that the mass of gel is difficult to remove from the container when needed. Moreover, the entire contents of the container must often be removed even if only a small portion of the gel is needed.
In accordance with one chilling technique to gel liquid photographic emulsions, the emulsion is carried on the top of a moving, continuous conveyor belt and glycol is sprayed on the bottom of the belt. As the belt reaches the emulsion discharge point and passes downwardly around the drive roller for movement along its return path, gelled emulsion is scraped off the belt and is broken into pieces. In another chilling process, the photographic emulsion is pumped through a scraped surface heat exchanger where the emulsion gels. The extrudate then passes out of the heat exchanger and breaks into pieces as it falls due to gravity.
Such processes have often not been found to be satisfactory, because they must process very large quantities of emulsion to be economically efficient, and, often, only relatively small amounts of gelled emulsion are needed at a given time. In addition, before an emulsion enters the chilling chamber, it may remain in a feed hopper for long periods of time which will cause settling and ripening and, as a result, a lack of homogeneity in the resulting emulsion gel. Further, some emulsions have too high a viscosity to be gelled with scraped surface heat exchangers. Although belt chilling devices do not encounter such viscosity problems, they are often too large to fit in existing facilities.